Polyolefin films have been used in packaging for many years. Many of these films are oriented films which have been stretched, typically in the machine direction, to provide films ideal for the packaging industry. Oriented films offer a wealth of advantageous properties, due to a change in the morphology of the film's molecular structure such as excellent mechanical properties, impermeability to moisture, high resistance to oils and fats and scratch resistance. These films are often low haze and high gloss and can be printed upon easily.
Machine Direction Oriented (MDO) films are, however, very difficult to seal by means of heat sealing. Exposing the highly oriented film to heat results in severe shrinkage of the film and rather low seal strength. The industry has generally solved this problem by designing complex multilayer film structures and using particular materials in the sealing layers of multilayer films.
It is known, for example, that homopolymers are characterised by low seal strength and their use in sealing layers is therefore avoided. Homopolymers often have higher melting points than copolymers making them harder to heat seal for example. Therefore multilayer film structures typically comprising a terpolymer, propylene random or propylene block copolymer are often used in oriented films. Biaxially oriented films are conventional made using polypropylene copolymers, for example.
Sealing is also a serious problem in more complex film operations such as in form fill and seal technology. In a typical form, fill and seal film sealing operation, the manufacturer has to seal different zones of the film. These zones may comprise 2, 3 or 4 film layers, especially where there are gussets present. Heat sealing the zones where 2 to 4 film thicknesses meet is difficult because the heat applied is often so great that the film can be damaged. If not enough heat is applied, a seal is not formed and the package integrity is at risk or the seal strength poor.
For oriented polyolefinic films therefore there are problems in heat sealing which need to be overcome. It would be useful to be able to seal homopolymer sealing layers or successfully seal multiple film layers without fear of damage.
The present inventors have realised that a potential solution to this problem is ultrasonic welding. Ultrasonic welding is an alternative sealing technology based on high frequency acoustic vibrations. Ultrasonic welding works by generating a very high voltage and converting that into high frequency vibrations by means of a converter (Piëzo elements).
A film surface to be sealed is exposed to the high frequency vibrations and that leads to interfilm and intermolecular friction between the film surface and the substrate to which the film is to be sealed. Heat is generated by friction in the sealing area and the sealing layer of the film and the substrate are sealed together.
Ultrasonic welding has traditionally been used in non polyolefinic environments. It has been used with laminates typically made of paper, aluminium or high-melting polymer films, such as polyethylene terephthalate. Its use in diaper technology is also known.
The use of ultrasonic welding of polyolefin films has been suggested but in few publications. In EP-A-1837162, a complex multilayer film is sealed using ultrasonic welding.
In EP-A-0332341, HDPE baby bottle liners are described which are stretched in the transverse direction and ultrasonically welded at one end. US2007/0215610 also mentions ultrasonic welding as one sealing option in a complex coextruded film for microwave applications.
The present inventors have realised that ultrasonic welding offers an ideal solution to the problem of heat sealing in machine direction oriented (MDO) polyolefin film. No one before has considered the use of ultrasonic welding in machine direction oriented polyolefin film. The present inventors show that this form of welding is also applicable in MDO polyolefin film, providing excellent seal strength, especially in the transverse direction.
The use of ultrasonic welding with MDO polyolefin films allows the formation of incredibly high seal strengths, in most cases higher than can be achieved using conventional heat sealing technology. This is especially true for transverse direction sealing.
Moreover due to the nature of the technology (i.e. no contact to hot seal bars) there is hardly any film shrinkage. The use of ultrasonic welding may allow more exact welding processes to take place and may allow the formation of a smooth weld line. This makes the seal less likely to break and avoids unsightly crinkles or tears. Film sealing can also be effected in the presence of contaminants on the sealing surface.
A further major benefit of the use of ultrasonic welding relates to a saving in raw material costs. When forming a seal between surfaces using conventional heat sealing, the layers may overlap by as much as 10 mm. Much of this overlap is therefore wasted film. With ultrasonic welding this overlap can be reduced to 6 mm. In the context of 2000 packages per hr, that adds up to a significant reduction in wastage. The process of the invention is therefore ideal for the manufacture of mass produced packaging such as heavy duty shipping bags.
The use of ultrasonic welding is therefore a significant leap forward for the MDO polyolefin film producer.